The short wavelength instrument for the polarization explorer balloon-borne experiment: Polarization modulation issues

In this paper we investigate the impact of using a polarization modulator in The Short Wavelength Instrument for the Polarization Explorer (SWIPE) of the large scale polarization explorer (LSPE). The experiment is optimized to measure the linear polarization of the cosmic microwave background at large angular scales during a circumpolar long-duration stratospheric balloon mission, and uses multimode bolometers cooled to.3 K. The 330 detectors cover three bands at 140, 220, and 240 GHz. Polarimetry is achieved by means of a large rotating half-wave plate (HWP) and a single wire-grid polarizer in front of the arrays. The polarization modulator is the first polarization-active component of the optical chain, reducing significantly the effect of instrumental polarization. A trade-off study comparing stepped versus spinning HWPs drives the choice towards the second. Modulating the cosmic microwave background polarization signal at four times the spin frequency moves it away from 1/f noise from the detectors and the residual atmosphere. The HWP is cooled to 1.6 K to reduce the background on the detectors. Furthermore, its polarized emission combined with the emission of the polarizer produces spurious signals modulated at 2f and 4f. The 4f component is synchronous with the signal of interest and has to characterized to be removed from cosmological data.